Add default security settings for public repositories

[snovikov]

⚠️ This is not a final fix, but rather an intermediate work-around.

Related bugs:

• [MAINT]: Public Repositories do not play well with Advanced Security settings integrations/terraform-provider-github#1419
• Provider update changes security_and_analysis block integrations/terraform-provider-github#1466

TL;DR

There are multiple issues with security_and_analysis. It is all the time set for public repositories, but have a buggy behaviour.
I case if you not set for public repo provider tries to remove settings, which results in error:

422 Advanced security is always available for public repos

In case if you set advanced_security to enabled, it will still produce error, but only on initial apply.